It is by far the biggest case of its kind to reach the courts since new regulations came into force in December extending sex discrimination rules to gays and lesbians.

Mr Lewis, a former global head of equity trading who is thought to have earned a pay and bonus package of more than [pounds sterling]1million a year, was dismissed in February after two complaints from male colleagues.

The case is believed to revolve mainly around an encounter between openly gay Mr Lewis and another male HSBC worker during office hours, which resulted in a complaint to the bank of " sexual harassment".

Mr Lewis's lawyer, Alison Downie, of solicitors Bindman and Partners, said that her client was dismissed "because HSBC discriminated against him because of his sexual orientation as a gay man".

His claim is likely to run to several million pounds.

Legal experts warn that the case could have wide and very costly repercussions for the whole City community - forcing big employers to rethink their approach to the way gay workers are treated in the workplace.

Employment lawyers argue that they are more likely to face allegations of harassment because of the "harmful prejudice" that homosexual men are likely to be more promiscuous than heterosexuals.

A source familiar with the Lewis case said: "Banks will need to radically rethink their codes of conduct and the way they discipline people in the light of the sexual orientation rules - just as they did when race and gender stereotypes were unpicked in the Seventies under the then-new race and sex discrimination laws. …